The applicant uses various topologies internally in order to design the effective cut-off frequency of a (low-pass) filter in a variable manner, that is, in an adjustable or switchable manner. For example, a filter bank which comprises several filters with different cut-off frequencies can be provided. The desired frequency band and/or the desired cut-off frequency are addressed by means of switches. However, the space requirement for solutions of this kind is large. The payload signal is often comparatively strongly attenuated by the switches used, and/or only payload signals of comparatively low power can be connected.
Preferred solutions for switchable filters should be capable of filter-internal and electronic switching. Such topologies are already used by the applicant, but in this case, for example, large reverse voltages can occur in switching diodes. As a result, the power consumption of the corresponding filter is limited to the breakdown voltage of the diode. A further problem may be that with these or other topologies, the power consumption of the overall circuit is limited by the maximal possible current consumption of the switch. Parasitic effects of the switches used or of their configuration are often not small enough by comparison with reactance values of resonator elements, so that the filter characteristic is negatively influenced by these effects.
Improved switching topologies should place reduced large-signal requirements on the switches and should be insensitive to switch parasitic effects.
US 2008/0129421 A1 describes an adjustable blocking filter with a transmission line and a switching element for the selective connection of the transmission line to ground. The switching element can comprise a PIN diode (Positive Intrinsic Negative Diode). Furthermore, a biasing circuit is provided for the switching element. It is proposed that the length of the transmission line is selected in such a manner that a first frequency is suppressed when the transmission line is connected to ground. By contrast, a second frequency is suppressed when the transmission line is not connected to ground. The transmission line can be implemented in the form of several transmission lines connected to the switching element in series.
The invention has the object among others of specifying a filter capable of switching in a filter-internal and electronic manner, especially for the gigahertz range, which achieves a high power consumption, which is as insensitive as possible to switch parasitic effects, and which places reduced large-signal requirements on the switches used.